A broadcast communication system employs a high-power amplifier as part of a signal transmission or transponder section of the system. Unfortunately, a high-power amplifier has non-linear distorting characteristics that cause distortion of the information signal which is amplified. The non-linear distorting characteristics of the high-power amplifier can impact the instantaneous amplitude and phase of the signal significantly. Only so much can be done to "linearize" a high-power amplifier due to cost and power dissipation.
Several known techniques are used to "pre-correct" an information signal in order to linearize the output of the amplifier. One of these techniques includes amplitude correction which produces a linear piece-wise pre-correction function which is correlated to the non-linear characteristics of the amplifier. The result is a piece-wise correction curve which approximates the ideal correction. The correction is added to the information signal.
It is also known to use an information signal to address a RAM which contains predetermined, pre-distorted versions of the information data values (see U.S. Pat. No. 4,291,277, to Davis et al). The pre-distorted version is supplied as a signal, in lieu of the actual information signal, to components (e.g., a quadrature mixer) which feed into the transmission amplifier. The substitute values in the RAM are updated via an analog comparison between a representation of the actual, desired data and a demodulated, "sliced" (or re-quantized) sample signal from the amplifier. The sample signal is provided for the comparison via suitable components (e.g., a quadrature demodulator) along a feed-back path.
However, the known system is limited by the values output by the quadrature mixer and the quadrature demodulator. For example, for a system employing a 16 QAM, quadrature demodulation of the sample signal is limited to one of four levels in each of the real (I) and imaginary (Q) axes. The known system does not have the ability to provide certain spectral corrections. Also, the analog comparison is limited by resolution of the analog signals. Specifically, the slicer of the demodulator does not provide information between sample periods (i.e., there is no useful information available). The know system cannot correct for out-of-band distortion. Thus, the know system cannot correct for the non-linearities induced by an amplifier onto a 32 bits-per-word PAM signal in an eight vestigial sideband system (8VSB). In particular, QAM is quantized to a limited set of levels in each of the I (real) and Q (imaginary) axes. In contrast, 8VSB is quantized in the I (real) axis, thus, the Q axis is infinitely variable.